Using online bubble size and total dissolved solids (TDS) measurements to investigate the performance of oxygen delignification, 2017PEERS

The main target of oxygen delignification is to continue delignification that started in cooking, i.e. remove a substantial fraction of the residual lignin using oxygen and alkali at a moderate temperature. Delignification with oxygen is a gentler way of reducing the Kappa number than extended cooking. Furthermore, the lower amount of lignin that goes in the pulp to bleaching, the smaller the organic waste water load from bleaching to the waste water plant and the less bleaching chemicals are needed.

Because oxygen is a weak oxidizing agent in its normal molecular state, it is necessary to promote its reaction by raising the temperature and/or providing alkaline conditions so the oxygen is more reactive in demolishing lignin structures. The organic substance dissolved in cooking creates side reactions during the oxygen stage. This causes additional oxygen consumption and degradation of cellulose chains. An increase in washing loss before the oxygen stage causes reductions in pulp viscosity, strength, and yield. Correspondingly, efficient post-oxygen washing is the key for low-cost bleaching.

We studied the performance of oxygen delignification by installing bubble size imaging systems and refractometers which measure dissolved dry solids in the oxygen stage feed and the top of the reactor. Based on these measurements, we gathered information about gas dispersion (bubble size distribution) and the behavior of dissolved matter in the hardwood mill’s oxygen delignification stage. We aimed to investigate the effect of different variables on the oxygen stage’s gas dispersion, Kappa reduction, yield, and pulp quality.  Gas dispersion improved (average bubble size decreased) when the chemical mixer speed (RPM) increased. Increasing both the mixer speed and the amount of oxygen indicated higher Kappa reduction and increased the amount of dissolved organic matter. 

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